With the receiver and TV off, the GPU probably can't read the EDID. It can't determine what the resolution of your TV is without the EDID, so it falls back to a safe default value. (Usually, EDIDs can be read when hardware is powered off and directly connected, but it's likely that the receiver doesn't pass through when it's powered off.)

You can work around this by setting custom modelines, or something that would probably be more convenient for you is to save your display's EDID to a file and then add it to your X configuration using the "CustomEDID" option.